Pickling of steel plates prior to nickel plating and coating

ABSTRACT

1. IN THE PRETREATMENT OF STEEL PLATE FOR DIRECT-ON WHITE ENAMELING WHEREIN THE STEEL PLATE IS ELECTROLYTICALLY DEGREASED AND PICKED, AND NICKLE PLATED BY ELECTRODESPOSITION ON THE SURFACE TO BE ENAMELLED IN A QUANTITY OF ABOUT 0.3 TO 0.7 G. PER SQUARE METER OF STEEL PLATE, THE IMPROVEMENT WHICH COMPRISES EMPLOYING STEEL HAVING A CARBON CONTENT OF ABOUT 0.043% AND EFFECTING THE ELECTROLYTIC PICKING TREATMENT AT A CURRENT DENSITY OF ABOUT 40 AMPERES/DM.2 IN AN ACID SOLUTION CONTAINING AT LEAST ONE OF THE SULFATE AND PHOSPHATE IONS FOR ABOUT 0.2 TO 3 MINUTES AT A TEMPERATURE OF ABOUT 40 TO 80* C.

United States Patent 3 840,441 PICKLING OF STEEL PLATES PRIOR TO NICKEL PLATING AND COATING Bruno Hees, Leverkuseu, Hans Hotfmanu, Leichlingen, and Gerhard Trogel, Bergisch Neukirchen, Germany, assignors to Bayer Rickmann GmbH, Cologne, German No l )rawing. Filed July 14, 1972, Ser. No. 271,898 Claims priority, application Germany, July 17, 1971, P 21 37 551.9 Int. Cl. C23]: 1/06, 5/08; C23d 5/00 US. Cl. 20434 Claim ABSTRACT OF THE DISCLOSURE In the pretreatment of steel plate for direct-on white enamelling wherein the steel plate is electrolytically degreased and pickled, nickel plated by electrodeposition and subsequently provided with a water soluble protective layer of at least one of a polyphosphate and an organic film-forming polymer, the improvement which comprises effecting the electrolytic pickling treatment at a current density of about 20 to 40 amperes/dm. in an acid solution containing at least one of sulfate and phosphate ions for about 0.2 to 3 minutes at a temperature of about 40 to 80 C. The nickel is electrolytically deposited in about 0.3 to 0.7 g./m. of steel plate.

The present invention relates to the pickling of steel plates prior to nickel plating and coating.

Over the years, the direct-on white enamelling of steel plate has been developed into a reliable process. Rationalization of the production cycle coupled with the introduction of continuous programmed production techniques has produced enamelled products of uniform quality coupled with a reduction in the enamelling costs. The production cycle in enamelling plants has been further simplified by preparing in the steel mill itself the band used to produce the blanks for direct-on white enamelling, for example by carrying out a suitable pickling treatment followed by nickel plating. The adhesion surface for direct-on white enamelling thus produced naturally has to be protected so that the layer applied is not damaged before any shaping work is carried out. In conventional band pretreatment techniques, such protection is provided by an electrolytically deposited layer of zinc (Belgian Patent Specification No. 557,963 and French Patent Specification No. 1,187,958).

The disadvantage of a metal protective layer of this kind lies in the complicated aftertreatment which is necessary and which requires the enamelling plant to erect another installation for several stages i.e. degreasing, rinsing, removing the protective layer with corrosives such as acid, e.g. hydrochloric acid, rinsing and neutralization.

Similarly, it is more rational to degrease and pickle metallic materials which have been formed into the required shape by drawing, pressing, bending or other nonchip-forming techniques and subsequently provided with a firmly adhering metallic or non-metallic layer, immediately after they have been produced and subsequently to provide them with a covering layer which, while protecting the surface, can nevertheless readily be removed. Coatings of metal obtained by electrodeposition or chemical deposition, stoving lacquers, dip lacquers and other plastics coatings also require as clean a substrate as possible in order to assure permanent protection of the surfaces. Accordingly, the problem of supplying the metal working industry with a starting material which can be directly used for subsequent coating is not confined to the enamelling industry.

3,840,441 Patented Oct. 8, 1974 The advantages are manifold; in addition to saving in materials by eliminating the need for additional derusting or for removing other corroded layers, pickling and the working up of eflluents which this involves can be carried out more economically in large installations.

A process is also known for pretreating steel plate (US. Patent Application Serial No. 864,516, filed Oct. 7, 1969) by degreasing, pickling, metallizing, neutralizing and drying, in which the pickling is carried out electrolytically, the plates to be pickled being treated at a current density of from 2 to 20 amperes/dm. for up to about 3 minutes at temperatures of from 40 to C. in an acid sulfatecontaining and/or phosphate-containing solution. Another characteristic feature of such baths is the presence of magnesium ions. This process was particularly developed for the direct-on white enamelling of steel plates. These pickling baths contain, more particularly, from 2 to 25% by weight of S0 ions, calculated in terms of sulfate; from 2 to 20% by weight of POQ- ions, calculated in terms of phosphate; and, preferably, from 1 to 5% by weight of magnesium, calcualted in terms of mag nesium hydrogen phosphate. With this pickling process in combination with an electrolytic degreasing operation, it is possible to shorten considerably the pretreatment time for steel plates for use in subsequent enamelling operations. After they have been pickled, the pretreated plates are provided by electrodeposition with a layer of nickel in a quantity of from about 0.3 to 0.7 g. per square meter of steel surface. This pretreatment technique provides an excellent substrate for direct-on enamelling, especially in the case of low-carbon steels.

This known process has been further improved by using a special nickel bath which enables the layer of nickel required for adhesion of the enamel to be deposited very quickly. The nickel sulfate-containing baths which have a nickel content of from 6 to 30 g. of nickel per liter of bath liquid are preferably used at pH values of from 3 to 6. In addition to buffer substances, chlorides are added to the baths in a quantity of from 3 to 20 g. of chlorine per liter of bath liquid, and magnesium ions in a quantity of from about 0.1 to 10 g. of magnesium per liter.

If after nickel plating the plates treated by the process described above are also provided with a protective layer which will withstand forming of the plates by rolling or pressing but which can nevertheless be readily removed, this process is also suitable for plate or band production.

According to suggested solutions set forth in US. applications Ser. Nos. 94,976, filed Mar. 31, 1970 and 98,- 104, filed Dec. 14, 1970, now pending, this coating is carried out either by masking with a water soluble polyphosphate layer or by applying a water-soluble film-forming organic polymer. In the first case use is made of watersoluble salts of polyphosphates having the general formula HO[HOPO ],,H, in which n is a larger number of at least 10 and preferably greater than 100. In the second case, polyethylene oxide, polyvinyl alcohol, carboxy methyl cellulose, hydroxyethylene cellulose and mixtures of these polymers may be used as the film-forming organic polymers. The organic polymers may also be combined with; the ployphosphates.

Covering the nickel adhesion layer with a readily removable protective film affords considerable advantages not only to the band or plate manufacturer but also to the processor. Plates or bands which have been pretreated in this way can be provided with a firmly adhering coating of enamel, metal or plastics immediately after forming without any need for a special pickling treatment, providing they are simple washed with water.

For the enamelling industry in particular, a fully continuous process is obtained by combining the pretreated sheet with direct-on white enamelling using modern application methods, such as electrostatic application, considerably reducing the amount of work compared with that which has been necessary in the past.

The present invention relates to an improved process for the pretreatment of steel plates for direct-on white enamelling which reliably provides satisfactory enamel finishes of high bond strength and which is suitable not only for open-coil decarbonized steel, but also for all the usual types of steel used in the enamelling industry.

The invention relates to a process for the pretreatment of steel plates for direct-on white enamelling in which, after they have been electrolytically degreased and pickled, the plates are nickel-plated by electrodeposition and then provided with a Water-soluble protective layer of a poly phosphate and/or an organic film-forming polymer, in which process the electrolytic pickling is carried out at a current density of from about 20 to 40 amperes/dm. in an acid sulfateand/or phosphate-containing solution for periods of from about 0.2 to 3 minutes at temperatures of about 40 to 80 C.

Electrolytic pickling is preferably followed by a galvanic nickel-plating operation in which nickel is deposited into the surface to be enamelled in a quantity of from about 0.3 to 0.7 g. of nickel per square meter of steel plate.

Surprisingly, it has now been found that normal steels can also be prepared for direct-on white enamelling by the pickling treatment according to the invention in such a way that a satisfactory, fault-free and firmly adhering enamel surface can be obtained.

The pickling treatment may be carried out by the dip technique, the plate to be pickled being connected as anode. The cathode material is not crucial to the success of the process and any plates of iron or other metallic materials may be used. There is never any need for the plates to be pickled for longer than 3 minutes. Pickling times of from about 0.2 to 3 minutes are adequate, depending upon the quanlity of the steel. In addition, the pickling time is governed by the current density, the composition of the bath and the temperature of the bath, and can thus be varied within certain limits. The bath temperature should be in the range from about 40 to 80 C., preferably from about 60 to 70 C.

The pickling bath should contain sulfate and/or phosphate ions. The SO ion concentration, calculated in terms of sulfate, should be from about 2 to 25% by weight, while the PO ion concentration, calculated in terms of phosphate, should be from about 2 to 20% by weight. The sulfate ions can be provided as sulfuric acid, alkali sulfates and/ or alkali hydrogen sulfates. In addition to the alkali metals, preferably sodium and/or potassium, it is also possible to use a magnesium sulfate or magnesiurn hydrogen sulfate. The PO ions can be provided in the form of phosphoric acid, alkali phosphates, magnesium phosphates, alkali hydrogen phosphates, alkali dihydrogen phosphates, magnesium hydrogen phosphate and/or magnesium dihydrogen phosphate. A bath containing from about 1 to 5% by weight of magnesium dihydrogen phosphate has proved to be particularly suitable. Phosphate-containing baths are generally advantageous because they have a favorable effect upon subsequent metallization of the plates to be enamelled, especially nickel-plating. Another advantage of phosphate-containing baths is that the iron which enters into solution during pickling is cathodically deposited. In this way, the service life of the baths is considerably lengthened.

The process according to the invention is suitable for all the usual types of steel used in the enamelling industry. In any event, firmly adhering direct-on enamel finishes are obtained by the pickling treatment according to the invention in combination with other necessary pretreatment stages.

In addition to the shortening of Working time and the saving in materials attributable to lower pickling-induced erosion, the prolonged service life of the baths alfords ancobalt ions, the conditions being adjusted so that approximately 0.3 to 0.7 g. of nickel and/or cobalt are deposited per square meter of steel surface.

Accordingly, nickel is applied in much smaller quantities than has hitherto been the case. No special measures have to be taken in carrying out the process according to this invention. The containers used in conventional nickel plating processes, for example plastic or even rubberized steel containers, can be used as the tanks. There is no need to regulate the temperature of the baths because deposition takes place sufficiently quickly at temperatures of from about 15 to 25 C. Although lower or higher temperatures are also possible, they are not economical.

The plate to be pretreated is connected as cathode in the known manner, while pure nickel or cobalt alloys thereof are used as anodes. Normally the bath is operated at average current densities of from about 0.3 to 0.8. amperes/dm. although, in special cases where particularly quick deposition is required, higher current densities may also be used but in such cases precautions must be taken to ensure that the workpieces do not catch fire. It is best to use either moving electrolytes or moving electrodes.

The baths are preferably prepared with water-soluble nickel and/or cobalt salts, such as the sulfates and/or chlorides. The concentration, calculated in terms of the metal, should be from about 8 to 30 g./ liter of bath liquid. It is preferable not to add any complex forming reagents to the baths. However, it may be of advantage to add boric acid or another bulfer substance, although such addition should be such that the pH value of the baths does not exceed about 7. In general, the baths can be operated at pH values of from about 7 to 1, pH values of from about 3.5 to 6 being preferred. Most of the metals should be present in the form of their sulfates. In order to accelerate anode solubility, the baths advantageously contain chlorides, from about 3 to 20 g. of Cl/ liter of bath liquid being a preferred chloride content. The chloride ions may be introduced in the form of the metal chlorides or in the form of alkali chlorides, including ammonium chloride. The presence of magnesium ions in the baths is also recommended to ensure suitable deposition of the nickel and/or cobalt. Extremely small quantitles of from about 0.1 to 10 mg./liter are suificient. Larger quantities are not harmful because the magnesium is not deposited with the nickel and/or cobalt and only indirectly influences the way in which the nickel and/or cobalt is deposited. The magnesium is introduced in the form of its Water soluble salts, preferably the sulfate and/ or chloride. The metallizing baths remain stable almost indefinitely providing impurities are not entrained in the bath upon the introduction of the workpieces. There is no need for regeneration because the nickel or cobalt concentration is automatically adjusted to the required level because as much nickel dissolves into the bath from the electrodes as is deposited onto the metal surfaces to be treated.

By combining the electrolytic pickling treatment according to the invention with the electrolytic metallizing treatment and, optionally, with the electrolytic degreasing treatment, the pretreatment time is considerably reduced compared with conventional processes. In many cases,

[fluctuations in the quality of the types of steel used can also be corrected by a fully electrolytic pretreatment of the plates, thus guaranteeing firm adhesion during the subsequent direct-on enamelling process. In addition to outstanding adhesion, a satisfactory enamel surface is also obtained in the same way. By virtue of the short residence times in the individual treatment stages, it is possible to pass even relatively small articles by means of the conveyor continuously through the treatment stages without any loss in capacity. Accordingly, there is no need in many cases for emptying and filling baskets or hangers with the articles which are to be enamelled.

More particularly, it is also possible for a fully electrolytic pretreatment to be directly followed by an electrostatic or electrophoretic application plant for the enamel frits. In this way, it is possible, providing the individual working processes are suitably arranged, to make the production cycle between shaping and enamelling substantially continuous.

The process according to the invention is illustrated by the following Example.

EXAMPLE A cold rolled steel plate with the following approximate analysis:

Electrolytic pickling (40 -a./dm. 70 C.; 77 g.

H 'PO /1+106 g. H SO 1) Cold rinsing l Electrolytic nickel plating (0.6 -a./dm. 20 C.; 80 g. NiSO -7 H O/ 1; 10 g. NH Cl/ 1; 20 g. MgSO -7 H O/ 1; 5 g. H 'BO 1) 1 Cold rinsing 1' Drying.

6 The plate pretreated in this Way was subsequently subjected to direct-on white en-amelling with a B-Ti-enamel of the following composition:

38.3% of Si0 2.0% of fluorine 21.8% of B 0 0.2% of MgO 15.3% of alkali oxide 3.3% of P 0 The quality of both the adhesion and the surface of the enamel were considerably better than that found in a comparison test where the electrolytic pickling was carried out with bath conditions of 10 a./dm. and 1'.

It will be appreciated that the instant specification and examples are set forth by Way of illustration and not limitation, and that various modifications and changes may be made without departing from the spirit and scope of the present invention.

What is claimed is:

1. In the pretreatment of steel plate for direct-on white enameling wherein the steel plate is electrolytically degreased and pickled, and nickel plated by electrodeposition on to the surface to be enamelled in a quantity of about 0.3 to 0.7 g. per square meter of steel plate, the improvement which comprises employing steel having a carbon content of about 0.043% and effecting the electrolytic pickling treatment at 'a current density of about 40 amperes/d-m. in an acid solution containing at least one of sulfate and phosphate ions for about 0.2 to 3 minutes at a temperature of about 40 to C.

References Cited UNITED STATES PATENTS 3,753,870 8/1973 Hotfmann et al. 20434 FOREIGN PATENTS 1,802,183 4/1970 Germany 204-34 F. C. EDMUNDSON, Primary Examiner 11.8. C1. X.R. 204--38 C 

1. IN THE PRETREATMENT OF STEEL PLATE FOR DIRECT-ON WHITE ENAMELING WHEREIN THE STEEL PLATE IS ELECTROLYTICALLY DEGREASED AND PICKED, AND NICKLE PLATED BY ELECTRODESPOSITION ON THE SURFACE TO BE ENAMELLED IN A QUANTITY OF ABOUT 0.3 TO 0.7 G. PER SQUARE METER OF STEEL PLATE, THE IMPROVEMENT WHICH COMPRISES EMPLOYING STEEL HAVING A CARBON CONTENT OF ABOUT 0.043% AND EFFECTING THE ELECTROLYTIC PICKING TREATMENT AT A CURRENT DENSITY OF ABOUT 40 AMPERES/DM.2 IN AN ACID SOLUTION CONTAINING AT LEAST ONE OF THE SULFATE AND PHOSPHATE IONS FOR ABOUT 0.2 TO 3 MINUTES AT A TEMPERATURE OF ABOUT 40 TO 80* C. 